A mutagenesis and screening program will be set up to obtain mutations in the circadian oscillator controlling the locomotor activity cycle in adult Drosophila. The location of the oscillator within the nervous system will be determined by the use of genetic mosaics. A search for specific cellular correlates of these mutations will be conducted by means of light and electron microscopy. The nature of a morphological abnormality in a specific neurosecretory cell group associated with a previously isolated arrhythmic mutation will likewise be investigated using light and electron microscopy. The relationship between this cell group and the circadian oscillator will be determined.